Sheet organic polymeric material is frequently used in place of metal in the preparation of vessels for gases and liquids under pressure. The strength of these sheet materials is increased by incorporating fibrous materials as layers of woven or nonwoven fabrics of organic or inorganic fibrous materials in the polymeric material. Threaded fittings are generally used to link these vessels to other operational equipment in a system. The fittings are normally attached from the low pressured side of the vessel wall.
Organic polymeric materials of such structures are usually thicker than metal when used in such applications. As a result, when conventional metal plugs and fluid conduits are threaded into passageways through such polymeric structures they frequently engage less than the full thicknesses of the structure. Forces on the plugs and conduits from fluid pressures often cause the polymeric materials to delaminate in the passageways on the pressure side of engaged threads, limiting the safe operating pressures. In addition, end plates of fiber-reinforced polymeric materials are often subject to surface delamination around threads accommodating pipe entries as a result of high tightening torques.
To accomodate high pressures in polymeric sheet structures, it has been proposed to use threaded inserts of metals in such structures as reinforcing plates which distribute fitting loads throughout the structure thickness. Such adaptations, however, are expensive and are more subject to corrosion than the structures they reinforce.